A new experimental medication, SIB-1508Y, that may help reduce the motor and cognitive symptoms of Parkinson's Disease (PD) is being examined in this research study. It has not been proven whether SIB-1508Y is safe and effective in treating PD. There may be risks involved in this treatment that have not been identified in the studies done so far. Animal studies suggest that this type of medication may help many symptoms of PD, but this will be the first time that SIB-1508Y is given to humans with PD. The purpose of this study is to assess the effect of SIB-1508Y (an experimental drug), compared with placebo (inactive drug or sugar pill), for use by patients who have been diagnosed with Parkinson's Disease and now require but are not yet receiving conventional PD therapies. We will be examining how safe SIB-1508Y is and how well tolerated the drug is. In addition, we will be looking at how SIB-1508Y affects the symptoms of PD, including both motor symptoms and symptoms affecting thought processes (e.g., attention span, memory, etc.).